A high power rotary transformer may be utilized to transform electrical power from one power grid operating at one frequency to a second power grid operating at a second frequency. U.S. Pat. No. 5,953,225 issued Sep. 14, 1999 to Larsen discloses a rotary transformer that includes a power recovery system to recover and apply to the transferee grid a power differential attributable to mechanical power channeled to a rotatable shaft of the rotary transformer.
Another such rotary transformer assembly is disclosed in Canadian patent application serial number 2,351,895 published Dec. 30, 2001 to Martin and Rehder. This rotary transformer assembly utilizes three phases of isolated bus duct passing through the center of the rotating shaft to connect the rotor of the rotary transformer to one of the two power systems between which the rotary transformer transforms the electrical power from one system through a stator to the other system operating at a slightly different frequency. The shaft assembly has a first upper shaft section or part containing radial holes at the top and bottom of this assembly through which lead ends of the bus duct radially pass 120 degrees from each other. The bus duct leads pass through exit holes in the upper portion of the upper shaft for connection with the collector rings to thereby connect this bus duct to a first power grid system. Similar exit holes are located at the bottom of this upper shaft section through which the lower bus duct leads radially pass for connection to the rotor winding of the rotary transformer. Both sets of exit holes are contained within the first upper shaft section. Below the first upper shaft section is a second lower shaft section. Both shaft sections are coupled together by a coupling flange. The rotor of the rotary transformer is supported on the lower shaft section.
The power recover system includes a drive motor connected to the upper shaft section between the two sets of leads of the bus duct. The motor applies torque to the upper shaft portion to recover power.
While the use of the bus duct passing through the upper shaft section of the rotating shaft has advantages associated with magnetic shielding, associated heating of the shaft and prevention of arcing, it should be understood that this bus duct assembly is fully contained within the upper shaft section between the collector ring and the upper end windings of the rotary. As a result, the full rated torque of the drive motor passes through a portion of the first upper shaft section between the two sets of radially extending bus duct exit holes cut through the shaft wall. The drive motor connection to the shaft is very close to the lower set of exit holes. Further this lower set of exit holes are located radially on the same plane. It should also be understood that for a high power application, the bus duct requires a diameter per phase in the order of 15 inches and the shaft has a diameter of about 54 inches. This leaves insufficient material in the shaft wall where the exit holes are cut to safely transmit the torque from the drive motor to the rotor of the rotary transformer. These holes create enormous stress concentrations in the shaft where the torquing movement of the drive motor is applied that must be compensated with very expensive and exotic steels. The steel shaft requires a relatively large thickness to counter the stress concentrations in the shaft to provide the strength necessary to transmit the torque. In some instances this may also increase the thickness of the shaft wall by up to 3 inches. These modifications to the upper shaft section are needed to compensate for a shaft that might otherwise be damaged, weakened, or broken during a short circuit of the drive motor or other short circuit conditions.